WC2 - Review the Introduction Review the Introduction What...

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Unformatted text preview: Review the Introduction Review the Introduction What is archeoastronomy ? What is cosmology ? What marked the beginning of modern astronomy ? Define a scientific theory Which are today the main two theoretical models of the Universe ? What is the meaning of the micro­universe ? What is a billionth of a billionth ? How many kilometers are in a light­minute ? Give three examples of international cooperation in the exploration of the Universe. How would you picture the Universe ? A possible picture of the Universe A possible picture of the Universe Observing the Macro­Universe (from Chapter 1) Observing light Optical telescopes How to See a Bee 1000 km Away Reduced visibility Seeing the Universe with new eyes Defining Light Defining Light Light is a form of electromagnetic radiation, which is an electric and magnetic disturbance that travels through space at the speed of 300,000 km/sec. It is produced by hot objects. Other types of electromagnetic radiation (in the order of increased energy): radio, microwaves, infrared(IR), visible, ultraviolet(UV), X and gamma. Properties of Light Properties of Light Light can be thought as a wave motion Reflection: When a beam of light hits the surface of a medium of higher density. Refraction: When a beam of light passes from air into glass or water it is bent. The shorter the wavelength the The distance between two successive wavecrests is the wavelength. The number of wavecrests per second is the frequency The height from crest to trough is the amplitude Waves interact through constructive/destructive interference greater the refraction and this provides the dispersion of light in prisms/lenses. Light Spectra Light Spectra The dispersion of light by a prism is called a continuous spectrum (like the rainbow). The blue component of light is refracted most and the red the least. The Sun’s spectrum contains dark lines which correspond to the light absorbed by atoms near its surface. (As you will see later, each atom or molecule has its own spectrum, a pattern of lines, which is related to the allowed electron transitions between its energy levels) Observations of the Absorbtion Spectrum of an astronomical object tell us the atomic/molecular composition of a star, or a planet, or a cloud of gas. Doppler Effect Doppler Effect If a source of light is receding, each successive wavecrest is emitted from a progressively greater distance and arrives later than it would if the source was stationary. A similar phenomenon occurs with sound waves, a pitch of an approaching source is higher than the pitch of a receding source. In astronomy, because the Doppler shift of the spectrum in the light of a receding star or galaxy is towards longer wavelengths (towards red), this effect is called red­shift (or blue­shift if the star is approaching us). Optical telescopes (I) Optical telescopes (I) With convex lenses objective focal point eyepiec e eye The larger the lens diameter the longer the focal length and the image obtained. With concave+convex reflective lenses mirrors (Cassegrain) primary mirror secondary mirror eyepiece Optical Telescopes (II) Optical Telescopes (II) Telescope Features Magnification (magnifying power) Light grasp is proportional to the aperture Resolving power Chromatic/ spherical aberation Mirrors easier shaped and mounted Visual observation replaced by photography and light analysis Optical Telescopes (III) Optical Telescopes (III) Telescopes: 1917 Mt.Wilson­Hooker 2.5m, 1948 Mt.Palomar­Hale 5.1m. Today: Mauna Kea­Keck 2x10m (with 36 hexagonal segments), Cerro Paranal–European Southern Observatory 4x8.2m, Hubble satellite (4m). Our eyes can see a bee at maximum 50m, while the 5m lens telescope at Mt.Palomar can see the bee at about 3km. A “double telescope” (or “interferometer”) can see the bee at 6000 km. ( H.Fizeau 1868) Reduced Visibility (I) Reduced Visibility (I) The atmosphere helped life to evolve on Earth, but it is the enemy of astronomy and astrophysics. Optical telescopes have to be at high altitude (Caucasus, Cerro Paranal, Mauna Kea). City lights can also influence observations (ex.Mt.Wilson­LA and Greenwich­London) Most other wavelengths are bad for ground observations. Even radio or microwaves (which can penetrate the atmosphere) can interfere with human signals. Reduced Visibility (II) Reduced Visibility (II) Altitude(km) 150 100 50 Molecular transitions Atomic transitions Nuclear transitions IR Visible UV X Gamma Radio Microwaves Getting the Full Picture Getting the Full Picture Optical telescopes gave just a small “window” to the Universe. Imagine how the world would look if our eyes could sense just red light ! Most non­optical telescopes must be used at high altitude The Universe contains much more than what we can see with optical telescopes. many abundant atoms have their strongest lines in UV dead stars which emit radio waves masses of gases moving at high speeds in the vicinity of stars or black holes produce X rays the center of our galaxy has often bursts of gamma rays. Seeing the Universe with New Eyes Seeing the Universe with New Eyes Radio telescopes: For high­resolution radio observations on uses Interferometers. the largest steerable dish is in Bonn (100 m) the largest fixed dish is in Arecibo (305 m) Ex. Socorro N.M. 27x25m dishes are equivalent to one dish of 36 km diameter. Balloons, high altitude planes or satellites, equipped with special detectors, are needed for observations on: microwaves, infrared (some IR, or heat, also penetrates the atmosphere), ultra­violet (UV), X and gamma rays. Radio­telescopes Radio­telescopes Arecibo Interferometer Observing Cosmic Particles Observing Cosmic Particles Apart from electromagnetic radiation one can obtain valuable information about the Universe by examining: meteors (hopefully not too big !) elementary particles, such as electrons, protons. Most are emitted by our Sun, others could come from other stars or can be byproducts of atmospheric collisions. Observing Gravitational Waves Observing Gravitational Waves According to Einstein’s general relativity gravitational waves slightly distort space. The Laser Interferometer Gravitational­wave Observatory (LIGO) built in Hanford (Washington), consists of two 4­kilometer long lasers beams which are built in such a way that in a certain place they add together destructively (i.e. the crests of one beam coincide with the troughs of the other beam). When a gravitational wave distorts space the two beams do not add up exactly and that creates a detectable signal. LIGO can detect spatial changes of the order of 10­16 centimeters. This kind of variation can come from many other phenomena such as microearthquakes, or the falling of trees. This is why LIGO has a twin experiment in Livingston (Louisiana) and only signals in both experiments are considered as corresponding to gravitational waves. ...
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This note was uploaded on 05/03/2011 for the course NATS 1740 taught by Professor Hall during the Spring '10 term at York University.

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